In modern microcircuits, the high-frequency capacitance of interlevel dielectrics is a critical parameter that must be understood for realization of high-speed (clock speed&gt;1 GHz) electronic devices. The characterization of the high-frequency dielectric properties of interlevel dielectrics is thereby crucial. To bridge electronic and optical gaps formerly encountered in the measurement of the dielectric constant in the GHz-THz frequency range, time-domain optoelectronic techniques that incorporate ultrashort laser pulses have been developed in recent years for microcircuit test devices.
For characterization of low dielectric constant materials before circuitization, however, a free-space, non-contact measurement is the most convenient and low-cost method. For this purpose, a time-domain coherent technique has been demonstrated in the far-infrared (FIR) range that has been shown to be a promising alternative to the conventional electronic or continuous wave method. Boosted by the rapid development of a compact and portable femtosecond (fs) laser system, time-domain FIR techniques using all room-temperature components have become attractive for a number of industrial applications, including, but not limited to, gas spectroscopy, measurement of conductivity, study of the dynamics of semiconductor materials, and measurement of water concentration in biological samples. With extremely flat frequency response, large dynamic range, and excellent signal-to-noise ratio (SNR), free-space electro-optic sampling (FS-EOS) has emerged as a coherent terahertz detection technique capable of detecting amplitude, phase, and spacial distribution information in a terahertz beam. For example, the refractive index and dielectric constant of thin films has been measured by inserting the film into a THz beam and comparing the Fourier transforms of the THz waveforms obtained with and without the thin film.
For free-space dielectric constant measurement of the film on a substrate, where the thickness of the film is much thinner than the wavelength of the applied electromagnetic (EM) waves, the free-space time-domain technique has a fundamental restriction. The principle of the coherent free-space technique for measurement of the dielectric constant is based on the evaluation of the relative phase shift due to the index of refraction, the index of refraction being the square-root of the dielectric constant. For a film thinner than the wavelength used for measurement, the visibility of the small phase shift in the waveform is difficult to obtain under realistic experimental conditions. For instance, for 100 GHz EM waves refracted through a one-micrometer film, a phase change on an order of only 10.sup.-3 radians is expected. This phase difference is extracted by comparing a first waveform refracted through the thin film on a substrate against a second, reference waveform reflected from the substrate without the film. Under most experimental conditions, this extraction is often difficult due to the experimental uncertainty between two separate measurements. Thus, it is highly desirable to measure the phase difference in a single measurement.